1. Field of the Invention
The invention relates to a short-circuit detecting circuit for detecting an output short-circuit of a class-D amplifier.
2. Description of the Related Art
In recent years, class-D amplifying circuits using a pulse width modulation system or the like are used in the field such as audio equipment. When an output of the class-D amplifying circuit is short-circuited, an apparatus such as a loudspeaker connected to the output or the class-D amplifying circuit itself is damaged. It is, therefore, necessary to detect the short-circuit of the output. For example, a pulse modulating type power amplifier for discriminating the short-circuit of the output from a state of an output signal and stopping the switching operation at the time of the output short-circuit, has been disclosed in Japanese patent Kokai No. 2007-235526 (Patent Document 1).
Since the class-D amplifying circuit is a simple buffer, it is ordinarily expected that a signal level of its output signal is the same as a signal level of an input signal. When an excessive current flows to the output of the class-D amplifying circuit due to the output short-circuit, however, a driving ability reaches a limit and the expected signal level is not outputted. In such a case, there is such a tendency that when it is expected that the output signal is at the high level, it is set to the low level, and when it is expected that the output signal is at the low level, it is set to the high level. In the short-circuit detecting circuit in the related art, by using the above tendency, the input signal and the output signal are monitored by an XOR (exclusive OR) circuit and, when the signal levels of both signals do not coincide, the short-circuit is detected.
In the class-D amplifying circuit in the related art, in order to prevent an excessive lead-through current from occurring in itself, an Hi-Z (high impedance) interval generating circuit is used, thereby setting the output upon transition of the signal level to Hi-Z.